The following meanings for the abbreviations used in this specification apply:
3GPP3rd Generation Partnership Project4PAM4 state pulse amplitude modulationACKAcknowledgementACLR Adjacent Channel Leakage RatioAMRAdaptive Multi-RateCM3Cubic MetricDPCCHDedicated Physical Control ChannelDPDCHDedicated Physical Data ChannelDTXDiscontinuous TransmissionE-DPCCHEnhanced Dedicated Physical Control ChannelE-DPDCHEnhanced Dedicated Physical Data ChannelEVMError Vector MagnitudeHS-DPCCHHigh Speed Dedicated Physical Control ChannelHSPAHigh Speed Packet AccessHSUPAHigh Speed Uplink Packet AccessLTELong Term EvolutionMPRMaximum Power ReductionNACKNegative AcknowledgementPAPower AmplifierPARPeak to Average RatioRFRadio FrequencyRRCRoot Raised CosineSESpurious EmissionSFSpreading FactorTTITransmission Time IntervalWCDMAWideband Code Division Multiple AccessWIMAXWorldwide Interoperability for Microwave AccessWLANWireless Local Area Network
Embodiments of the present invention relate to controlling transmit power of a network element, such as a user equipment (UE). Particular embodiments of the present invention relate to calculation of maximum power reduction (MPR) for 4PAM modulated high speed uplink packet access (HSUPA) signals, which is a measure that approximates non-linearities of a power amplifier and which is required in determination of power amplifier back off.
Power amplifiers (PA) of transmitters are in general non-linear, which causes distortion that increases the Error Vector Magnitude (EVM) and spurious emissions (SE). Signals that have a higher Peak to Average Ratio (PAR) will also have a higher linearity requirement for the PA. There are two possibilities to meet the higher linearity requirement: either the PA is designed to be more linear or the operating point of the existing PA has to be set so that the signals do not get distorted. As the PAs become more expensive from the point of view of manufacturing cost and power consumption when the linearity of the PA is increased, it is often more desirable to use the existing PA designs. The distortion (and thus EVM and SE) of the PA can be controlled by adjusting its operating point. Typically, when PAR of the base band signal is increased, the operating point of the PA has to be adjusted towards a more linear region in order to maintain EVM and ACLR. This adjustment can be done by increasing an output back-off of the PA.
A practical example of the increase in PAR is in HSPA wherein the HS-DPCCH and E-DPDCH channels are multiplexed into the Release 99 channels. For high power levels this will cause the power amplifier to work in a non-linear region, thus increasing ACLR and spectrum mask leakage. In order to tackle this problem and to enable use of PAs that have been designed for the Release 99, the standard allows the UE to reduce the maximum transmit power when HS-DPCCH and/or E-DCH are present. The allowed reduction of MPR has been introduced already in Release 5 and remains valid for the next releases as well.
The calculation of the maximum power reduction (MPR) involves the cubic metric (CM3), which was taken into use by 3GPP as it was found that PAR is not the best metric to estimate the actual impact of the PA and a table-based approach, which was used in previous 3GPP Releases, would have been too complicated. The cubic metric value approximates the 3rd order non-linearity of the PA better than PAR and thus enables a generalisation of the amount of PA back-off allowed to fulfill the ACLR requirements.
There are two fundamental problems in the cubic metric calculation. First, the cubic metric is computationally rather complex, especially when 4PAM is used, and it has to be calculated every time the channel gain factors change. In principle the calculation for every TTI would be enough, but due to the HS-DPCCH transmission (ACKTNACK or DTX) it may change on every slot and therefore CM3 must be determined for every slot. Furthermore, if E-DPDCH scaling occurs, the cubic metric has to be re-calculated within the current slot before the data is to be transmitted. Secondly, there is no time to calculate the cubic metric after the RRC filtering (i.e. pulse-shaping) as some time has to be allocated for the transmitter to set up the back off. The samples at the output of the pulse shaping filter would be the correct ones to use. However, due to the latency and computational complexity, a method that allows calculation of CM3 prior to the pulse shaping filter is preferable.
For HSUPA, 3GPP TS 25.101 Chapter 6.2.2. sets the requirements for the Cubic Metric and MPR. How the Cubic Metric/MPR is actually computed/estimated is vendor dependent, but all of the methods have to be basically based on the beta-values of each uplink channel, i.e. channel gain factors, which are described in 3GPP TS 25-213.
The formula for calculating the Cubic Metric (CM) as defined in chapter 6.2.2 of 3GPP TS 25.101 is as follows:CM=CEIL{[20*log 10((v_norm3)rms)−20*log 10((v_norm_ref3)rms)]/k,0.5}where                CEIL {x, 0.5} means rounding upwards to closest 0.5 dB, i.e. CM□[0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5]        k is 1.85 for signals where all channelisations codes meet the following criteria: CSF,N where N<SF/2        k is 1.56 for signals were any channelisations codes meet the following criteria: CSF,N where N≧SF/2        v_norm is the normalised voltage waveform of the input signal        v_norm_ref is the normalised voltage waveform of the reference signal (12.2 kbps AMR Speech) and        20*log 10((v_norm_ref3)rms)=1.52 dB        
One method to estimate MPR/cubic metric is illustrated in US-A1-2010/153049 “Calculating a Non-Linearity Metric”. This method defines all possible uplink signal states before spreading, scrambling and pulse shaping operations and estimates MPR and Cubic Metric using the defined signal states and the above-listed MPR/cubic metric calculation formula given in chapter 6.2.2 of 3GPP TS 25.101 mentioned above.
Even though the method illustrated in US2010-A1-153049 simplifies the theoretical cubic metric/MPR calculation, it is still rather computationally complex. A complex calculation may cause problems for HSUPA signals having 4PAM modulation due to the tight timing requirements of MPR and cubic metric calculation.
Therefore a simpler solution to define the cubic metric and MPR for HSUPA signal with 4PAM is needed to be able calculate MPR/cubic metric as specified in 3GPP TS 25.101.